Slideshow: Adhesives Make BMW's All-Carbon Composite Body-in-White

When we told you about BMW unveiling its i3 all-electric car last summer, we knew it would contain carbon fiber composites in the structure, not just the panels. What might not have been obvious, though, was how much and exactly where. And we didn't yet know how.

The new BMW i3's entire body-in-white is made of carbon fiber composites, a first in automotive mass production. It turns out that its construction has been made possible by Dow Automotive Systems' Betaforce structural adhesives, Eugenio Toccalino, Dow's director of strategic marketing, told Design News in an email.

BMW's intelligently designed LifeDrive architecture for its BMW i cars consists of two elements: the aluminum bodyshell, called the Drive module; and the carbon composite Life module. All the heavy stuff, like the motor and battery, is in the lower Drive module. The lightweight carbon fiber Life module is the car's passenger cell that sits on top, compensating for all that weight. You can see illustrations of this architecture for the i3 here, and a video of the same architecture, shown for the i8 hybrid, here.

Click on the BMW i3 below to start the slideshow.

The new BMW i3's entire body-in-white is made of carbon fiber composites, a first in automotive mass production. This has been made possible by Dow Automotive Systems' Betaforce structural adhesives, which can be optimized for commercial production according to manufacturer specifications. (Source: BMW)

As we told you last fall, Betaforce and Betamate structural adhesives were designed to join carbon composites not only to each other, but also to metals. At that time, Dow Automotive's senior market development manager, Frank Billotto, shared slides from a presentation he'd been giving. These showed that the adhesives were designed with characteristics that make the composite body-in-white possible. They include a continuous bond line, cohesive joining of different surfaces, managing dissimilar expansion rates in different materials, and coping with the galvanic corrosion that can occur if aluminum fasteners used in metal structures contact composites. The adhesives also help increase body stiffness and durability, and cure at room temperature with cycle times of around one minute.

You can see how fast those cycle times really are in this video of the i3's Life module being assembled by robots. It's all done in the first eight minutes. In some of the closeups, you can see heat being applied after gluing and clamping, and it looked to me as if it cured in under a minute.

Toccalino gave us some additional details as to how this was all accomplished, especially the fast cure rates. First, Betaforce adhesives are designed to reach the specific heat-triggered, accelerated cure profile of the process selected by an automotive OEM. "In general, [these] structural adhesives also have been designed to achieve a rapid strength build-up in heat-accelerated curing processes, generally determining adhesive temperatures of approximately 90C to 110C, resulting in handling strength within minutes," he told us. Heat can be applied by ovens, microwave heating the adhesive, infrared irradiation, or induction heating the substrate to transfer heat to the adhesives, among other techniques.

Adhesive cure rates can be accelerated further without compromising their failure modes by using Dow Automotive's Betawipe activators, which are tailor-made to improve surface adhesion, Toccalino told us. Interestingly, the end-cure properties, such as shear strength, of a particular adhesive formulation are independent of its cure rate.

Those properties also include the combination of high elongation at break with high modulus. This makes the adhesives especially suited for bonding carbon fiber composites, because high modulus gives the assembly full stiffness, and high elongation provides improved dynamic loading and energy uptake, says Toccalino. The adhesives' modulus stability over a wide temperature range also contributes to long-term stability and durability of the bonded assembly. The adhesives themselves are optimized to meet OEM specifications of a 15-year vehicle life. Once fully cured, they also meet all OEM environmental durability specifications.

The adhesives can be applied to multiple materials because they bridge differentials in the thermal elongation coefficients of dissimilar material assemblies across operational temperature ranges, particularly critical between carbon composites and aluminum. Aside from the adhesives' mechanical properties and cure kinetic profile, the use of Betaprime primers and Betawipe surface activators develop perfect and robust adhesion on complex substrates like carbon composites, says Toccalino. "For example, we offer one grade of Betaforce that has been specifically developed to reduce or even eliminate surface preparation in multimaterial assembly."

As we've discussed, and as readers have often commented, repairing components made of carbon fiber composites can be expensive, if it's even possible at all. Dow Automotive has tried to make that easier by providing a longer open-time version to extend handling and working times for repairs.

Ann, absolutely fascinating video. I use adhesives on a daily basis adhering a metal bracket to a piece of tempered glass. We use LOCTITE two-part 5600 for that assembly AND a LOCTITE promoter. I am constantly amazed at the performance characteristics of the RTV being used. Your comment about "tack time" is well taken. We know if we do not install bracket on glass in under one minute we will have issues with separation. By modifying the mixture ratio we could lengthen the tack time and had to find, by experimentation, a suitable set time relative to logistics of the process. There were several things I noticed about the video. These are as follows:

1.) Excellent and profound use of robotic systems.

2.) Compared to assembly in our country, very very few individuals working on the assembly lines. We could hear them in the video but "hands off" during processes.

3.) Noise levels were extremely low.

4.) Robotic systems were six-axis SCARA types.

I think every high school senior needs to look at this video to discover the factories of the future will not, for the most part, employ thousands of people. They need specialized education, post high school and probably college, to survive in manufacturing if that's where they want to be.

I didn't realize that either, NZgorski. What's interesting is that, either later in this video, or in another I saw, the assembly of the carbon fiber panels require some human application of adhesives.

NZgorski, isn't that video amazing? Among other things, it gives an indication of how sophisticated automotive manufacturing robots have become.Thanks for the info on your company's services and a refresher on one side of the fasteners vs adhesives debate. We've discussed the topic several times in DN, and the answer seems to depend in part on the materials to be fastened/attached and the application. Degree of recyclability is also an issue that can affect the choice.

That video was incredible to watch, thank you for sharing. You are right, the bond time for the adhesive seemed to be very quick and it's amazing how quick the entire process moves along.

At Strouse we die cut adhesives and foams for the automotive industry and just yesterday put up a new blog post on the advantages of using adhesives to replace fasteners and bond different materials. Just some of the quick advantages of adhesives over fasteners:

Lightweight so veheicles are more fuel efficient and produce less emissions.

Fasteners can leave materials exposed and corrode over time.

Adhesives also create less stress on the materials that are being bonded. It spreads the stress over the surface of the part where the adhesive is applied as opposed to the concentrated spots where fasteners are placed.

University of Southampton researchers have come up with a way to 3D print transparent optical fibers like those used in fiber-optic telecommunications cables, potentially boosting frequency and reducing loss.

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